Flamethrower > minigun. More points and kills. I'd recommend not using either the flame or mini unless there's not very many people left alive though, otherwise you don't get much out of it. TBH both of those are only good on huge crowds.
Here's what I like to use:
Viper + Spec 2 Slot + Flame thrower + Minigun
Flame thrower is an epic clutching weapon especially when you're doing 4 man and you want a heck load of points. You go outside and wait for a mob to come out and you profit $$
I use the minigun like a machine gun. I wait for zombies to pile up at the door, and spray 'em like fuck